The use of glucose sensors in the medical field is widespread. The regular monitoring of blood glucose levels of diabetic patients at home as well as the use of glucose monitoring in intensive care units are two primary examples. The usual aim in developing a glucose sensor is to produce a digital electronic signal, which is proportional to the glucose concentration. The sensor usually comprises two main components, a chemical or biological part that reacts or complexes with the glucose (ideally specifically) to form new chemical or biological products or changes in energy that can be detected by means of the second component, a transducer. The chemical/biological component can be said to act as a receptor/indicator for glucose. A variety of transduction methods can be used including electrochemical (such as potentiometric, amperometric, conductimetric, impedimetric), optical, calorimetric and acoustic. After transduction the signal is usually converted to an electronic digital signal.
Since the signal generated by the chemical/biological reaction with the analyte is usually dependent not only on the concentration of the analyte but also on the characteristics of the sensor itself, such sensors usually require calibration before they can be utilised quantitatively. The way in which the signal varies with the analyte concentration determines the shape of the calibration curve (signal versus analyte concentration) and may define the number of calibration points. Typical calibration curves can be straight line, exponential, s-shaped etc and the principal of calibration applies to all methodologies of transduction for chemical or biological sensors.
Ideally, the sensor should be calibrated just before its use since some sensor characteristics that can affect the calibration curve vary with time (ageing effect). It is often the case that the time between sensor manufacture and use can be many months, so calibration at the point of manufacture can lead to inaccuracies in the end result. This means that the attendant clinician or nurse, or the home user, will be required to perform the calibration themselves. In order to maximise user compliance, the calibration process should be simple to perform, ideally invisible to the person performing the calibration, and be quickly completed.
Typical calibration techniques currently in use require the sensor to be inserted into three solutions having differing, but known, glucose concentrations (one of which may be zero), a reading to be taken for each solution, and a calibration curve to be generated. Since glucose has a tendency to degrade when sterilised in an aqueous solution, commonly the glucose solutions must be made up at the time of calibration by the addition of solid glucose to water or a solution such as saline. The entire calibration process is therefore both complicated and time consuming. Calibration of many currently available glucose sensors could 25 minutes or more to complete. There is therefore a need for a more rapid calibration technique for a glucose sensor, in order to improve user compliance.